Creating A Dichotomous Keyby Cassie Zanca

The students will explore the benefits of creatingdichotomous keys as a means of identifying anorganism or object.

GOAL

The student will create a dichotomous key toidentify seashells.

A dichotomous key is a tool that allows theuser to determine the identity of items and organismsin the natural world. It is the most widely used form ofclassification in the biological sciences because it offersthe user a quick and easy way of identifying unknownorganisms. Keys consist of a series of choices that leadthe user to the correct name of a given item.“Dichotomous” means “divided into two parts.” That iswhy dichotomous keys always give two choices in eachstep. In each step, the user is presented with twostatements based on characteristics of the organism. Ifthe user makes the correct choice every time, thename of the organism will be revealed at the end.

There are two kinds of descriptions that mightbe presented to the user of a dichotomous key:qualitative and quantitative descriptions. Qualitativedescriptions concern the physical attributes, orqualities, of the item being classified. Examples ofqualitative descriptions are such phrases as “containsgreen striations on top surface” or “feels slick on

bottom surface.” Quantitative descriptions concernvalues that correspond with the item being classified.Examples of quantitative descriptions are such phrasesas “has 10 striations on top surface,” “has 8 legs,” or“weighs 5 grams”. Knowing the difference betweenthese two types of descriptions can be immenselybeneficial for creators and users of dichotomous keys.

There are two ways to set up a dichotomouskey. One way is to present the two choices together,and the other way is to group by relationships. Whenthe dichotomous key is set up by presenting the twochoices together, it is easy to distinguish between them.However, relationships between various characteristicsare not emphasized. When the dichotomous key isgrouped by relationships, the choices are separated,yet it is easy to see the relationships between them.While this method may prove to be more difficult toconstruct, many users prefer it because it gives themmore information.

OBJECTIVE

BACKGROUND

BENCHMARKS (Louisiana)

MIDDLE SCHOOL BENCHMARKS HIGH SCHOOL BENCHMARKSSI-M-A1-A7 SI-H-A1-A6SI-M-B1-B7 SI-H-B1-B5

LS-H-C4,C6

MIDDLE SCHOOL STANDARDSUC&P- Systems, order & organizationLS - Diversity & adaptations of organisms

NATIONAL SCIENCE STANDARDSHIGH SCHOOL STANDARDSUC&P - Systems, order, & organization

1 Education on the Halfshell: Creating A Dichotomous Key Louisiana Sea Grant College Program

STUDENT INSTRUCTIONSYou will create a dichotomous key in order to identify oyster shells or pictures of shells. Your teacher will provideyou with the samples to identify. Before writing your dichotomous key, read the background information on shellsprovided by your teacher, as it will aid you in key construction. Continue to write the key until each individualshell can be identified. Make an answer key to accompany your dichotomous key.

ANSWER KEYEach student or student group will submit a dichotomous key and answer key; therefore, each answer key will bedifferent.

• Create a concept map from the key.• Students will design a dichotomous key using a particular item (examples: coke tabs, ketchup bottles,

paper clips, other shells) and be graded on accuracy and completeness.

EXTENSION STARTERS• Have students create a field guide using dichotomous keys to help with identification of a particular

item (examples: plants on the school campus, or macro-invertebrates in a local stream, etc). Givestudents examples of field guides to generate ideas.

• Using the key they have created, develop an outline of the format used in their Englishclass, that students will understand what the divisions in an outline represent and the relationships

among the terms.

In this activity, the teacher will need or collect five to six oyster shells for each student or student group. If oystershells are not readily available, download the pictures of the oyster shells from the website. Enclosing thepictures in sheet protectors may be a good idea. From these shells or pictures, the students can now create theirown dichotomous keys.

ALTERNATIVE ASSESSMENT IDEAS

Timme, Stephen, 1991, Association for Biology Laboratory Education website, How to Construct and Use aDichotomous Key, accessed 02/16/01, http://www.zoo.utoronto.ca/able/volumes/vol-12/7-timme/7-timme.htmDescription: An excellent web-based activity on the construction and use of a dichotomous key that also describesthe use of a dichotomous key in the field and provides a key for prairie plants.

Frontier High School, Red Rock, OK, The Dichotomous Key, accessed 02/16/01, http://pc65.frontier.osrhe.edu/hs/science/hbotkey.htmDescription: Provides instructions on the two methods of constructing a dichotomous key as well as several onlinedichotomous keys. Grade level: High School.

Detka, Jon , California State University at Monteray Bay, Designing and Using a Dichotomous Key, accessed 02/16/01, http://www.monterey.edu/students/Students_D-H/detkajon/world/ron/dichotdesign.htmlDescription: Students first construct a simple dichotomous key and then use a basic key to identify some of thenative plants and the most unwanted invasive weeds of California. Grade level: 3-5.

Santa Cruz Productions, Wastewater Filamentous Bacteria Dichotomous Key, accessed 02/16/01, http://home1.gte.net/vsjslsk1/gramstainflowchart.htmDescription: A completely web-based dichotomous key designed to assist students in identifying wastewaterbacteria.

RESOURCES & WEBLINKS

TEACHER PREPARATION

2 Education on the Halfshell: Creating A Dichotomous Key Louisiana Sea Grant College Program

MOLLUSKS AND THEIR SHELLS

3 Education on the Halfshell: Creating A Dichotomous Key Louisiana Sea Grant College Program

Shells are the hard exteriors of living organisms and are formed for two main purposes. Firstly,they protect the organisms from predators and environmental stressors. Secondly, they providerigidity and shape. One well-known group of animals that secrete shells is called mollusks.

There are eight classes of mollusks grouped according to thecharacteristics of their shells. Single-shelled organisms are calledGastropods. A snail is a gastropod, with a single shell thatspirals outward and to one side as it grows. The coiling may beeither to the left or right. The coiling can be indicative ofpaleoenvironmental conditions. Global temperature changes aresaid to be the reason for reversal of the coiling; therefore, somegastropods can be important in the field of geologic dating.

The shelled animals that are studied in this unit are called Bivalvia and include oysters, clams,scallops, and mussels. They have two valves that enclose the soft tissue of the mantle and otherinternal organs. The shells are hinged together.

Another well known class is the Cephalopoda, which include octopus, squids, and nautilus. Mostcephalopods have a reduced shell or are shell-less. The nautilus is the only one with a completelydeveloped shell. The squid and cuttlefish have a small internal shell, and the octopus completelylacks a shell.

There are five other classes of mollusks not as well known. They includeCaudofoveata, Aplacophora, Monoplacophora, Polyplacophora, andScaphopoda. Caudofoveata are shell-less wormlike mollusks that live inburrows on the deep-sea floor. The Aplacophorans also lack a shell andlive in the depths of the ocean, physically attach to and or eat relatives ofthe jellyfish and coral. Monoplacopharans have a caplike shell and areless than 3 cm long, they are found in the ocean depths.Polyplacophorans or chitons are flat long mollusks with a shell made ofeight plates. Chitons live on rocky shorelines and graze on plants.Scaphopods also known as tusk shells, and have a long tube-shapedshell tapered at one end and open on both ends. These mollusks burrowinto the sand.

Mollusk shells are made from calcium that is extracted from the watersin which they live or from the food the mollusks eat. There are twocommon forms of calcium carbonate, aragonite and calcite. They differin crystal shape, but their chemical formula is the same. The denser ofthe two forms, calcite, is found in the hard, outer shell of the oyster.Whether calcium carbonate becomes aragonite or calcite depends onthe “seed” crystals growth pattern.

Many mollusk shells are lined with aragonite. Abalones and some oysters, including Louisianaoysters, are examples. The substance they produce is called “mother of pearl” or nacre.Nacre is what gives oysters and abalones the shine on the insides of their shells.

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ANATOMY OF THE OYSTER SHELL

4 Education on the Halfshell: Creating A Dichotomous Key Louisiana Sea Grant College Program

• All species of oysters share the same common anatomical parts. That is, thereis a right shell and a left shell. The right shell is the top shell. The left shell isthe one attached to the substrate and is called the bottom shell or cup. The left shellis usually more curved or cup-shaped than the right shell.

• Oysters belong to the group Bivalvia and have two shells hinged together. Theend with the hinge is referred to as the anterior end. The opposite end is referredto as the posterior end.

• The mantle produces layers of tissue to form the shell. Calcium carbonate composes the chalky middle layer. The innermost layer is the nacre.

• The umbo is the portion of the shell that formed when the oyster was a veligerlarva. It is on the anterior end of the shell near the hinge. One can see concentricgrowth lines around the umbo. These are not uniformly secreted since they can beinfluenced by environmental changes, such as temperature, food, stress, anddisease. These factors affect the oyster’s ability to produce more shell tissue.

• The hinge is the area where the two valve join together. It consists of three parts.On the hinge there are small projections, hinge teeth, that align the valve andenable it to close correctly. Oysters have very small hinge teeth. Most bivalves havemore prominent teeth that can be used to help identify the species. The hinge hastwo ligaments. The external ligament is flexible and provides the axis of movement for the two halves of the shell. The internal ligament is called the resilium.When the adductor muscle of the oyster is relaxed, the external ligament andresilium work together to open the shell. To close its shell, the oyster contracts itsadductor muscle.

nacre

posterior end

umbo (hinge)

anterior end

LEFT SHELL

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Creating A Dichotomous Key: Pointers

5 Education on the Halfshell: Creating A Dichotomous Key Louisiana Sea Grant College Program

When constructing a dichotomous key, one should keep certain things in mind.• Use easily identifiable characteristics such as two valves verses one valve, instead of using

variable characteristics such as color.• Use quantitative measurements, such as 6” long or a size range of 3-6” long, rather than

qualitative terms suchas “large” or “tall”.• Use characteristics that are generally available in nature or to the user, rather than

seasonal characteristics or those that can only be observed easily by scientists withspecial equipment.

• Each choice should have a positive response greater than a negative one. For instance,it “has” a characteristic rather than it “does not have” one.

• Whenever possible, begin both choices with the same word.• Wherever applicable, begin different pairs of choices with different words.• Always precede descriptive terms with the name of the part that you are describing.

While you are constructing the key, you must be as specific as possible so that the userdoes not become confused. If the two choices are not precise, a user can easily choose the wrongone, and may get the wrong classification or have to start over.

You must be alert to the wording of every choice. Doing so may save you from having torewrite the entry in order to arrive at the correct answer. Be cautious and strive to get the mostcorrect classifying characteristics possible.

Another helpful idea is to get several people to use the key to see if their answers matchwith yours. If all arrive at the same answer, it is safer to assume that the key was written cor-rectly. If there is a discrepancy among the users, analyze the key together and compare yourchoices to see where the differences occurred. After you have discussed the reasons for thedifferent choices, modify the key to adjust for any problems.

When using a key, the following may be helpful.• Always read and consider both choices, even if the first one seems to be appropriate.

Jumping to conclusions may lead to the wrong classification of the item.• Always understand the meaning of the words used in each choice. Define the

term. If you are not sure of the meaning, look it up in a dictionary. Neverguess, as this could also lead to the wrong classification of the item.

• When there are measurements given in the choices, use the appropriate measuring tools oradjust them to match your own set of tools. For example, if a key measurement is given incentimeters but your ruler is divided into inches, convert the centimeter measurement intoinches. Do not approximate and do not guess. Measure.

• If you are classifying a living or once-living thing, do not base your conclusion on a singleobservation. Living things almost always exhibit variability, so it is better to study manyspecimens in order to be sure that your results are representative of the majority.

• If you are left with two possible answers, read the description of both and decide which oneseems to fit your specimen more precisely.

• When you have “keyed out” all of the shells, do not accept your answers to be correct.Check the description of each organism in order to see that it appropriately matches theorganism. If it does not, then an error was made somewhere in key development.

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Fossil Oyster Pictures for Creatinga Dichotomous Key

Cubitostrea perplicata

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Pycnodonte (Gigantostrea) trigonalis

Lopha johnsoni

Cubitostrea sellaeformis

Hyotissa vicksburgensis

Fossil Oyster Pictures for Creating a Dichotomous Key

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Ostrea (Turkostrea) duvali

Agerostrea falcata

Exogyra costata

Flemingostrea subspatulata

Pycnodonte mutabilis

Fossil Oyster Pictures for Creating a Dichotomous Key

Ostrea (Turkostrea) multilirata

Crassostrea gigantissima

Pycnodonta (Phygraea) pulaskensis

American OysterCrassostrea virginica